There is a general interest in the art to characterize the index of refraction (also called refractive index, or RI) of the materials used in optical and opto-electronic systems. These materials include liquids, gels, and solids.
Index matching fluids (IMF) have been widely used in fiber optic applications in order to reduce the reflection losses between optical components. They can take the form of liquids or gels, each having a substantial range of viscosity. The refractive index is one of the most important properties of an 1 MF, and for some optical systems, knowledge of the precise value is absolutely critical. Precise determination of the refractive index of liquid substances is also desired in non-optical fields, such as in the food and oil industries since the refractive index is often correlated with other properties and since it can often provide an easier way of indirectly measuring those other properties.
A refractometer, a prism coupler, and an ellipsometer are three traditional apparatuses for measuring refractive index. In a refractometer, light is passed through a transparent block of solid material and the distance between the exit spot and a reference point is determined to evaluate the refractive index of the material. In a prism coupler, a prism is placed on top of a layer of the material to be measured, and the critical insertion angles, i.e., angles of the optical tunneling for various modes at the interface between the prism and the layer, are used to evaluate the refractive index of the thin film material. An ellipsometer relies on the polarization dependence of the light reflecting at an interface, and on the phase change, which depends on the refractive index of the material when the light travels through a layer of the material. More recently, a number of interferometric methods have been developed and applied to measure refractive index and/or its changes for various materials. All these methods require a solid piece of material, or at least a cured gel-like layer of material.